Arthurian knight possessed after festival (7)
I believe the answer is:
galahad
'arthurian knight' is the definition.
(I've seen this before)
'possessed after festival' is the wordplay.
'possessed' becomes 'had' (to have something is to possess it).
'after' is a charade indicator (letters next to each other).
'festival' becomes 'gala' (a gala is a festival or party).
'had' put after 'gala' is 'GALAHAD'.
(Other definitions for galahad that I've seen before include "Chivalrous knight" , "Son of 9" , "principled chap" , "extremely honourable man" , "Noble knight" .)